PN junctions and diodes

In the diode equation I = Is {exp(qV / nkT) − 1}, what current flows when a large reverse voltage is applied (within the range where no breakdown occurs)?

  1. AIt becomes infinite
  2. BIt keeps increasing in proportion to exp(qV / nkT)
  3. CIt stays nearly constant at about −Is (the reverse saturation current)
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Answer: C. It stays nearly constant at about −Is (the reverse saturation current)

With a large reverse voltage (V a large negative value), exp(qV/nkT) is nearly 0, so I ≈ −Is. Only a tiny, nearly constant reverse saturation current flows. In real diodes, increasing the voltage further leads to breakdown.

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More PN junctions and diodes questions

  1. What is the purpose of a diode (flyback diode) connected in parallel with an inductive load such as a relay coil?
  2. In a forward-biased PN junction, which capacitance arises from the storage of injected minority carriers?
  3. How does the depletion width of a one-sided abrupt junction depend on the reverse bias voltage V (when V is much larger than the built-in potential)?
  4. Why are termination structures such as guard rings placed at the edge (periphery) of a high-voltage PN junction?
  5. In a simple voltage regulator using a Zener diode, what is the role of the resistor placed in series with the diode?